The first monster I was scared of was Dory the Friendly Seamonster. (Super Mario 64, Hazy Maze Cave)
I used to go on the elevator. See her head in the distance, and would reset the game in fear.
The eel, well, he was only scary on the first star Jolly Roger Bay map. Forget wha it is called.
And I know this isn't a sea monster, but when I had 120 stars a long time ago, I was scared of Yoshi's footsteps, thinking it was Bowser. So I erased that profile. And to this day, I still have 2 stars I am too lazy too forget.
*Fixed*